The invention relates generally to steam turbines. More particularly, the invention relates to a steam turbine stage with integral covered buckets of different materials.
The steam flow path of a steam turbine is generally formed by a stationary casing and a rotor. In this configuration, a number of stationary vanes are attached to the casing in a circumferential array and extend inward into the steam flow path. Similarly, a number of rotating buckets are attached to a rotating shaft of the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating buckets are arranged in alternating rows so that a row of vanes and the immediately downstream row of buckets form a stage. The vanes serve to direct the flow of steam such that it enters the downstream row of buckets at the correct angle. Airfoils of the buckets extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached thereto.
As the steam flows through the steam turbine, its pressure drops through each succeeding stage until the desired discharge pressure is achieved. Thus, steam properties such as temperature, pressure, velocity and moisture content vary from row to row as the steam expands through the flow path. Consequently, each bucket row employs buckets having an airfoil shape that is shaped for the steam conditions associated with that row. In addition to airfoil shape, the buckets terminate in integral covers that are sized and positioned to maintain contact with the cover of an adjacent bucket in a row when assembled and during use. There are two reasons for this structure. First, the continually contacting covers increase steam path performance by reducing and/or eliminating gaps between adjacent buckets and the cover and vane interface. Second, buckets that do not have continual cover contact with adjacent buckets become ‘freestanding’, which leads to failure. Maintaining continual cover contact is a design challenge for applications in excess of, for example, about 975° F. due to the onset of long-term creep of the vane and/or rotor interface. Current approaches use advanced materials, such as a nickel-based alloys for integral covered buckets or stainless steel alloy buckets with peened (i.e., separated) covers in regions of the steam turbine where creep is a limiting factor.